Python Tutorial

Python Join Lists

Join two lists with +, a loop, or extend().

Join with +

Create a new list from two lists.

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)

extend()

Add list2 onto list1 in place.

list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)

📘 Real-World Deep Dive

Joining a list with a separator is a daily operation but easy to do badly (<code>+</code> in a loop vs. <code>str.join</code> once). Knowing the canonical patterns makes output code short and fast.

Real-Life Scenario

Compose a few different "join-like" outputs: CSV row, SQL <code>IN (...)</code>, shell command, and multi-line block — all from the same list of identifiers.

Real-Life Example

import shlex

ids = ["user-1", "user-2", "user-3"]

csv = ",".join(ids)
sql_in = ", ".join(f"'{x.replace(chr(39), chr(39)*2)}'" for x in ids)
shell = " ".join(shlex.quote(x) for x in ids)
block = "\n".join(f"  {x}" for x in ids)

print("csv :", csv)
print("sql :", f"SELECT * FROM users WHERE id IN ({sql_in});")
print("sh  :", shell)
print("block:")
print(block)

Expected Output

csv : user-1,user-2,user-3
sql : SELECT * FROM users WHERE id IN ('user-1', 'user-2', 'user-3');
sh  : 'user-1' 'user-2' 'user-3'
block:
  user-1
  user-2
  user-3

Common mistakes

Concatenating strings in a loop with += is O(n²); use "".join(xs).
Naive CSV join misses quoting/escaping — use the csv module for serious work.
shlex.quote is the safe way to embed filenames into shell commands; don't roll your own.

🚀 Performance & Best Practices

"".join(parts) is C-fast for thousands of fragments; reduce(operator.add, parts) is not.
Use io.StringIO when each fragment needs non-trivial formatting.
Pre-build the join template to skip str.format overhead.

🧪 Try It Yourself

Add support for multi-character separators per join style.
Implement join_with_and(items) returning "a, b, and c".
For SQL, parse into sqlalchemy.sql.quoted_name instead of hand-rolling.

FAQ: Python Join Lists

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What is Python Join Lists?

Python Join Lists is a Python Tutorial lesson that explains join lists python in Python. Join two lists with +, a loop, or extend(). Copy the samples and run them in the Python editor. It is written for beginners who want a clear definition and working examples.

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What are common mistakes with join lists python?

Common join lists python mistakes include wrong syntax, mixing types, and skipping practice. Work through this Python Tutorial chapter in order, run every example, and check the output before moving on.

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